Process for the preparation of 3-axial hydroxy steroids

ABSTRACT

It has been found that steroids having hydroxy groups in the equatorial orientation may be converted into corresponding steroids having said hydroxy group in the corresponding axial position. In particular, 3 Beta -hydroxy A/B trans steroids may be converted into the corresponding 3 Alpha -hydroxy steroids. In the process of the present invention a 3 Beta -hydroxy steroid is reacted with triphenyl phosphine in the presence of dialkyl azodicarboxylate and a carboxylic acid to yield the corresponding 3 Alpha -carboxylic acid ester which may, if desired, be saponified to the 3 Alpha -hydroxy A/B trans steroid. The method provides a simple route to the formation of important physiologically active steroids using the more readily available 3 Beta -hydroxy isomer. Included among these compounds is androsterone.

United States Patent n91 Hose [ PROCESS FOR THE PREPARATION OF l-AXIAL HYDROXY STEROIDS [76] Inventor: Ajay K. Bose, 248 Morris Avenue,

Mountain Lakes. NJ. 07046 [22] Filed: July 17, i972 [Zl] Appl. No: 272.23l

[56] References Cited UNITED STATES PATENTS 3/]972 Kruger 260/3974 7/1974 Gregory et al. 260/3974 OTHER PU BLlCATlONS Mitsunohu et al. Bull. Chem. Soc. Japan vol. 44 pp. 3428-3430 I971).

l l Nov. 11, 1975 Primary E.\'aminerElbert L. Roberts Attorney, Agent. or Firm-Behr & Woodbridge [57] ABSTRACT It has been found that steroids having hydroxy groups in the equatorial orientation may be converted into corresponding steroids having said hydroxy group in the corresponding axial position.

In particular. 3B-hydroxy A/B trans steroids may be converted into the corresponding 3a hydroxy steroids. in the process of the present invention a BB-hydroxy steroid is reacted with triphenyl phosphine in the presence of dialkyl azodicarboxylate and a carboxylic acid to yield the corresponding 3a-carboxylic acid ester which may. if desired. be saponified to the 3a-hydroxy A/B trans steroid. The method provides a simple route to the formation of important physiologically active steroids using the more readily available 3B-hydroxy isomer. Included among these compounds is androste rone.

l0 Claims, No Drawings PROCESS FOR THE PREPARATION OF 3-AXIAL I-IYDROXY STEROIDS FIELD OF THE INVENTION Conversion of steroid compounds having equatorial hydroxy groups to the corresponding steroid having axial hydroxy groups.

DESCRIPTION OF THE PRIOR ART It is well known that the physiological activity of steroids is often highly dependent upon the stereochemical orientation of a single group in the molecule. It is also known that the most active compound is often not the one thermodynamically preferred. Thus, compounds having the critical group in the thermodynamically favored equatorial position are often more readily available, than the sometimes more biologically active axial form. Methods are known to the art for the conversion of steroids having an equatorial hydroxy group to the corresponding steroids having an axial hydroxyl group. The classical method for such conversion is reported by Chang and Blickenstaff (J. Amer. Chem. Soc.; 80, 2906 (1958)). This method requires the conversion of the hydroxy group in question into the corresponding tosylate which is then transformed into the corresponding formate by reaction with dimethylformamide during which transformation inversion occurs. While the yields in this prior art mode are fairly acceptable (Circa. 73%) the reaction time of approximately 45 hours, presents substantial problems where these conversions are to be carried out on an industrial scale.

It has been reported (Mitsunobu and Eguchi, Bull. Chem. Soc. Japan, 44, 3428 (1971)) that various acids and acidic compounds such as benzoic acid will react with secondary alcohols in the presence of triphenyl phosphine and diethyl azodicarboxylate to yield esters with stereochemical inversion. However, the teaching of Mitsunobu and Eguchi would appear to indicate that inversion occurs regardless of the initial stereochemical orientation of the starting alcohol.

SUMMARY OF THE INVENTION Steroids having hydroxyl groups in the equatorial position may be converted into the corresponding steroids having said hydroxyl group in the axial position. In the process of this invention the steroid in question, for example, a 3B-hydroxy A/B trans steroid is reacted with triphenyl phosphine and diethyl azodicarboxylate in the presence of a carboxylic acid to form the corresponding 3a-steroid ester. It is our interesting and surprising finding that this reaction is a one-way reaction. In our hands, the corresponding Sol-hydroxy A/B trans steroid could not be converted to the corresponding SIB-steroid ester.

The ester is then converted to the corresponding alcohol by saponification in the usual manner. Where it is desired to carry out the reaction of the present invention with compounds having ester groups in other portions of the molecule and it is desired to preserve them in the final product, the acid utilized in the reaction of the present invention is trifluoroacetic acid. which forms esters which are readily hydrolyzed under conditions which would leave other esters such as acetates, benzoates and the like intact.

It is a central feature of the present invention that the process may be carried out in the presence of sterically hindered hydroxyl groups which will not be affected by it. For example, 1-, 7-, 12-, and l7-hydroxy groups.

It should be noted, however, that where the ring juncture between the A and B rings of the steroid is A/B trans and where a A double bond is present, the designation of the S-equatorial hydroxy group is 35, however, where the said ring juncture is A/B cis, then the 3-equatorial hydroxy group is designated as 30:.

DESCRIPTION OF THE PREFERRED EMBODIMENTS In the process of the present invention there is used as starting material a steroid having a secondary hydroxyl group attached to one of the rings of the steroid nucleus. The orientation of this hydroxyl group shall be equatorial. Its location on the nucleus is not critical, provided that the position is not sterically hindered. Thus, the hydroxyl group may be in the 2, 3, 11,15 and 16 positions. Whether the orientation of these hydroxyl groups is designated as alpha or beta will be determined by the stereochemical orientation of the particular ring to which said hydroxyl is attached.

The hydroxy steroid having a hydroxyl in the equatorial orientation, is then taken up in a mixture of triphenyl phosphine, diloweralkyl, suitably dimethyl, diethyl or dipropyl azodicarboxylate, and an organic acid, suitably a carboxylic acid or a phenol. There may be utilized an alkyl, haloalkyl, polyhaloalkyl, aryl, alkaryl or aralkyl acid. Especially suitable, though by no means limited thereto, are acids wherein the alkyl moiety contains l5 carbon atoms and the aryl moiety is phenyl such as formic acid, acetic acid, propionic acid, benzoic acid, toluic acids, chloroacetic acid, trifluoroacetic acid, phenylacetic acid, phenol and substituted phenols such as nitro, halo and alkyl substituted phenols. Where it is desired to produce a final product having an ester grouping attached to the inverted position, the acid corresponding to that ester may be utilized. in the case of phenols, the compounds produced are of course phenyl ethers rather than esters. It is preferred to carry out the reaction in a solvent. Non-hydroxylic solvents such as ether or tetrahydrofuran are preferred. The reaction may be carried out at between about 0C to about 50C, preferably however the reaction is carried out at room temperature. In one modification of the reaction, the steroid, triphenyl phosphine, acid and solvent are mixed and the dialkyl, suitably diethyl azodicarboxylate added thereto, suitably in a portion of the solvent utilized. However, no particular preference is to be attached to this mode of proceeding.

It is important to utilize dry reagents and to exclude moisture from the reaction vessel by methods well known in the art in order to prevent sice reactions involving the triphenyl phosphine. The reaction is carried out utilizing one (1) mole of reagent (triphenyl phosphine, acid, and diethyl azodicarboxylate) per mole of monohydroxy steroid. Thus where for example, the steroid contains more than one hydroxyl group or keto group, irrespective of whether said hydroxyl group will be subject to stereochemical inversion or not, it is necessary to add 1 mole of triphenyl phosphine for each additional hydroxy or keto group per mole of steroid.

Provided the foregoing minimal ratios are satisfied excess quantities of reagents may be used. However consideration should be given to the inconvenience of removing unreacted triphenyl phosphine from the reaction mixture.

The reactants are agitated, suitably at room temperature for from about 8 to about 18 hours, the solvent removed, suitably under reduced pressure, and the residue purified. it is especially preferred to purify by column chromatography, using, for example, a Florisil column.

Where it is desired to maintain a previously located ester group on the steroid, the acid of choice is trifluoroacetic acid.

Similarly where it is desired to form a phenyl ether, in place of a carboxylic acid there is used, depending on the desired end-product, a phenol or substituted phenol for example, phenol itself, an alkyl phenol, such as cresol, an alkoxy phenol such as methoxy phenol or a halo phenol such as p-bromo phenol.

Where the ultimate product is desired to be the corresponding axial hydroxy steroid, the ester prepared in the foregoing step is subjected to hydrolysis in the usual manner. For example, the ester is taken up in aqueous alkanol, suitably aqueous methanol, and heated under reflux for from about I to about 20 hours with a strong base, for example an alkali metal hydroxide or alkoxide, such as sodium or potassium hydroxide, methoxide, or ethoxide or the like, using from about 1.1 to about 2 moles of alkali for each ester moiety per mole of steroid.

Where a phenol has been used in the inversion reaction, cleavage of the ether is achieved by methods well known in the art, for example by treatment with boron trichloride or boron tribromide.

Where it is desired to maintain ester groups upon the portion of the steroid other than the hydroxy moiety subjected to inversion, and where therefore the corresponding trifluoroacetate has been prepared, the trifluoroacetyl steroid is taken up in aqueous alkanol, suitably aqueous methanol (preferably one part of water in 10 parts of methanol), agitated at room temperature for from about to about 15, suitably about hours, in the presence of an excess of mild inorganic base suitably an alkali metal carbonate, and the product removed from the reaction mixture by extraction, suitably with methylene chloride or the like, which is then worked up in the usual manner.

It is our surprising discovery that the process of the present invention is stereo specific from unhindered to hindered orientations of a hydroxyl group only where, as in the case of steroids, the conformation of the asymmetric center is stereochemically locked with respect to the rest of the molecule. Thus, for example, where a mixture of 4-cisand 4-trans-t-butylcyclohexanol, which are eonformationally mobile, was subjected to the reaction of the present invention utilizing formic acid as the acid, there was obtained a mixture in the same proportions, of the corresponding formates. On the other hand, mixtures of SB-cholestanol and 3aeholestanol yielded a mixture of 3a-hydroxy and 3aformyl steroids, and Sa-cholestanol itself was unchanged when subjected to the process of the present invention. It has further been found that the presence ofa double bond at the 5 position of the steroid nucleus does not appear to affect the progress of the present invention. Thus, cholesterol is readily converted into the corresponding Ba-benzoate. Similarly, it has been found that 33, l6a-dihydroxy-A -androstene is converted, in a reaction in accordance with the process of the present invention, utilizing benzoic acid, to the cor responding 3oz, l6B-dibenzoyloxy-A -androstene.

Utilizing the process of the present invention the readily obtainable SB-hydroxy-Sa-pregnane-l1,20- dione, and 2l-acetoxy-3B-hydroxy-5apregnane-20- dione, are converted into the corresponding 30:- hydroxy analogs of said starting steroids which are components of anesthetic agent, CT-l34l (D. Campbell, et al, Brit. J. Anaesth., 43, I4 (l971)). The first component may be prepared using any of the acids listed as suitable in the process of the present invention, however, the second component having a 2l-acetoxy group, is, suitably, reacted with trifluoroacetic acid, in order to preserve the 2l-acetoxy moiety.

The currently used methods of the preparation of 3- hydroxy steroids generally lead to a mixture of 3B- and 3oz-hydroxy derivatives. Subjecting this mixture to the process of the present invention, will yield, in the first instance, a mixture of the axial 3-ester and hydroxy steroids which may then, upon saponifieation, yield the pure axial 3-hydroxy compound which, in the case of A/B trans steroids would have the Sol-orientation, and in the case of A/B cis steroids, the 3B-orientation is obtained. For example, methyl cholate was converted to methyl 3B-formyl-7a, l Zmdihydroxy cholanate.

Among the physiologically active 3a-hydroxy steroids which may be prepared from the corresponding 3B-isomers by the method of the present invention may be listed:

la-methyl androstane-3a,l 7B-diol 3a-hydroxy-5a-androstan-l7-one l6,16-difluoroandrostane-Sa,l 7B-diol l6,16-difluoroandrostane-Iia-ol17-one 16,16-difluoro-l7a-methylandrostane-3a,l 7B-diol l6,l6-difluoro-l7a-ethinylandrostane-3a,l 7 B-diol 5a-androstane-3a,l 7B-diol-l7-( l'-ethoxyl methyl)-ethyl ether 5a-androstane-3a,l 7B-diol-l7-(cyclopent-l '-aryl)- ether 20-hydroxymethyl-Sfi-pregnane-3a,l 113,21 -triol Compounds which may be used as starting materials for the preparation of these Zia-steroids may be included in the general formula The term loweralkyl is used in this specification is to be interpreted as alkyl having 1-5 carbon atoms.

EXAMPLE 1 Formation of 3a-Cholestanol from 3B-Cholestanol Equimolar quantities of 3B-cholestanol (1.94 g.), benzoic acid (0.61 g), triphenyl phosphine (1.31 g.) and diethyl azodicarboxylate (0.87 g.) were added to 60 ml. of dry tetrahydrofuran. The reaction mixture was protected from moisture with a guard tube (calcium chloride) and stirred overnight. The solvent was then removed under reduced pressure and the semisolid residue was dissolved in boiling methanol-ethyl acetate mixture. Upon cooling slowly 3B-cholestanol benzoate crystallized out, 1.7 g. (70%),mp 98-l00, with satisfactory infrared and NMR spectra.

Using the same reaction conditions on 30:- cholestanol, only unchanged starting material was recovered.

The thus obtained 3a-cholestanol benzoate (400 mg.) was taken up in aqueous methanol (1:10) containing potassium hydroxide (200 mg.). The mixture was heated under reflux for hours, cooled, acidified with dilute aqueous hydrochloric acid, extracted with methylene chloride and the methylene chloride extract washed with aqueous sodium carbonate (.1 N). The methylene chloride extract was dried over sodium sulfate, the solvent evaporated and the residue recrystallized from ethanol/ethyl acetate to yield, upon cooling, 3oz-cholestanol mp. 186 with satisfactory infrared and NMR spectra.

In accordance with ths above procedure but using the 3a-formate of androsterone in place of 3acholestanol benzoate, there is obtained androsterone.

In accordance with the foregoing procedure but where in place of diethyl azodicarboxylate there is used dimethyl-, dipropylor dibutylazodicarboxylate the same result is obtained.

EXAMPLE [I Inversion Reaction on 5a-Androstane-3B-ol-17-one Equimolar quantities of Sa-androstane-3B-ol-l7-0ne (0.51 g; 0.002 mole), formic acid (0.092 g; 0.002 mole), diethyl azodicarboxylate (0.348 g; 0.002 mole) and 2 moles of triphenyl phosphine (0.524 g; 0.004 mole) were added to dry tetrahydrofuran (40 ml.). The reaction mixture was protected from moisture with a guard tube (calcium chloride) and stirred overnight at room temperature. The solvent was then removed under reduced pressure and the semi-solid passed through a Florisil column using hexane/benzene (65:35) as eluent, first 100 ml. of solvent mixture on evaporation gave 3oz-formate of androsterone, 0.590 g. (approx. 92%),mp 181 (lit mp 181-181.5 with satisfactory infrared and NMR spectra. Direct comparison of this material with an authentic sample (mixture mp undepressed; superposable infrared spectra) showed identity.

In accordance with the foregoing procedure, but where, in place of formic acid, there is utilized acetic acid, propionic acid, benzoic acid, phenyl acetic acid, trifluoroacetic acid, phenol, or p-bromophenol, there are obtained the corresponding 3a-acetate, 3apropionate, Sa-benzoate, 3a-phenyl acetate, 30:- trifluoroacetate, Zia-phenyl ether, and 301-pbromophenyl ether of androsterone.

In accordance with the foregoing procedure, but where in place of 5a-androstane-3B-ol-3-one, there is utilized 3B-hydroxy-5a-pregnane-11,20-dione, 21- acetoxy-3B-hydroxy-Sa-pregnane-l 1,20-dione, there are obtained the corresponding 3a-esters and ethers of 3a-hydroxy-5a-pregnane-l1,20-dione and 21-acetoxy- 3-hydroxy-5a-pregnane-1 1,20-dione.

EXAMPLE III 3a-Trifluoroacetoxy-Sa-Cholestane Equimolar quantities of 3a-cholestanol (0.95 gms.), trifluoroacetic acid (0.57 gms.), triphenyl phosphine (1.31 gms.), and diethyl azodicarboxylate (0.87 gms.) were dissolved in dry tetrahydrofuran (50 ml.), stirred overnight at room temperature and solvent was removed under reduced pressure and the residue was passed through a short Florisil column using hexane as the eluent. Evaporation of the hexane fraction gave 2 grns. (82%) of 3a-trifluoro-acetoxy-Sa-cholestane. lnfrared and NMR spectra were satisfactory.

In accordance with the foregoing procedure but utilizing 2 l-acetoxy-3B-hydroxy-5a-pregnane-1 1,20- dione, and methyl cholate in place of SB-cholestanol there is obtained the corresponding 2l-acetoxy-3atrifluoroacetoxy-Sa-pregnane-11,20-dione and methyl- 3B-trifluoroacetoxy-7a, 1 Za-dihydroxy-cholanate.

EXAMPLE IV 3a-Cholestanol Sodium carbonate (0.5 gms.) is dissolved in 1 ml. water and methanol (10 ml.), 3a-trifluoroacetoxy-3acholestane is added thereto and the mixture stirred at room temperature for 10 hours.

The mixture is extracted with methylene chloride and the organic layer washed with water and dried (anhydrous sodium sulfate). Removal of the solvent under reduced pressure yields 3a-cholestanol, identical with an authentic sample in melting point and spectra characteristics. The yield is essentially quantitative.

In accordance with the foregoing procedure but where in place of 3a-trifluoroacetoxy cholestanol there is utilized 21-acetoxy-3a-trifluoroacetoxy-5apregnane-1l,20dione or methyl-SB-trifluoroacetoxy- 7a,l2a-dihydroxy cholanate, there is obtained 21- acetoxy-3 a-hydroxy-Sa-pregnanel 1,20-dione, and methyl 3a, 1 2a-t rihydroxy cholante.

EXAMPLE V 3a-p-Bromophenoxy-Sa-Cholestane 3a-Cholestanol (3.88 g.), triphenyl phosphine (2.62 g.), p-bromophenol (1.73 g.), and diethyl azodicarboxylate (1.74 g.) in ml. of tetrahydrofuran were allowed to react for 5 days at room temperature. After removal of the solvent under reduced pressure the residue was chromatographed over a short alumina column. The earlier fractions gave a crystalline solid which was further purified by recrystallization from methanol/ethyl acetate. The 3a-ether, mp 109, was obtained in about 60% yield. The structure of this compound was verified by infrared, NMR and mass spectroscopy.

EXAM PLE Vl 3a-Cholestanol Sa-p-Bromophenoxy-Sa-cholestane is treated with boron trichloride in dry tetrahydrofuran. Work-up in the usual manner yields 3a-cholestanol.

I claim:

1. A method of inverting the stereochemical orientation of the 3-hydroxy moiety of 3-equatorial hydroxy steroids which comprises reacting said 3-equatorial hydroxy steroid selected from the group of compounds of the formula:

wherein T is a carbon-carbon single bond or carbon-carbon double bond, provided that where T is a carbon-carbon single bond, the hydrogen at 5 is X is fluorine, chlorine or bromine R, is hydroxy, alkanoyl, R O-CH,.C=O

where R is alkanoyl, aroyl, aralkanoyl. alkaroyl or hydroxy, lower alkyl, lower alkynyl, cyclo lower alkenyloxy, (lower alkoxy lower alkyl) lower alkoxy, (hydroxy lower alkyl) hydroxy lower alkyl, acetonidodihydroxy lower alkyl, R, is hydrogen, and R and R when taken together are O, with triphenyl phosphone, di-

loweralkyl diazocarboxylate; and a member selected from the group consisting of alkyl, haloalkyl, polyhaloalkyl, aryl, alkaryl, and aralkyl carboxylic acids, phenols and halophenols,

whereby there is formed a compound selected from the group consisting of the corresponding Zia-steroid esters and Boa-steroid phenyl ethers.

2. A process according to claim 1 further comprising the step of reacting an ester produced in accordance with the process of claim 1 with a saponifying agent.

3. A process according to claim 2 wherein the saponifying agent is a strong base.

4. A process according to claim 3 wherein the base is an alkali metal hydroxide or alkali metal alkoxide in an alkanol.

S. A process according to claim 4 additionally comprising the step of isolating the thus produced 30:- hydroxy steroid therefrom.

6. A process according to claim 1 additionally comprising the step of reacting a phenyl ether produced in accordance with the process of claim 1 with boron trichloride or boron tribromide.

7. A process according to claim 1 wherein where the R is alkanoyl, aroyl, aralkanoyl, or alkanoyl, the carboxylic acid is trifluoroacetic acid.

8. A process according to claim 7 additionally comprising the step of reacting the product of claim 7 with an alkaline metal carbonate in aqueous alkanol.

9. A process according to claim 1 which comprises sequentially reacting 2l-acetoxy-3[3-hydroxy-Sapregnane-11,20-dione with:

a. trifluoroacetic acid in the presence of triphenyl phosphine and diethyl azodicarboxylate, and

b. sodium carbonate in aqueous methanol.

10. A process according to claim 5 which comprises the sequential steps of reacting Sa-androstane-3B-ol l7-one with:

a. formic acid in the presence of triphenyl phosphine and diethyl diazocarboxylate and b. potassium hydroxide in aqueous methanol. 

1. A METHOD OF INVERTING THE STEREOCHEMICAL ORIENTATION OF THE O-HYDROXY MOIETY OF 3-EQUOTRIAL HYDROXY STERIODS WHICH COMPRISES REACTING SAID 3-EQUATORIAL HYDROXY STERIOD SELECTED FROM THE GROUP OF COMPOUNDS OF THE FORMULA:
 2. A process according to claim 1 further comprising the step of reacting an ester produced in accordance with the process of claim 1 with a saponifying agent.
 3. A process according to claim 2 wherein the saponifying agent is a strong base.
 4. A process according to claim 3 wherein the base is an alkali metal hydroxide or alkali metal alkoxide in an alkanol.
 5. A process according to claim 4 additionally comprising the step of isolating the thus produced 3 Alpha -hydroxy steroid therefrom.
 6. A process according to claim 1 additionally comprising the step of reacting a phenyl ether produced in accordance with the process of claim 1 with boron trichloride or boron tribromide.
 7. A process according to claim 1 wherein where the R3 is alkanoyl, aroyl, aralkanoyl, or alkanoyl, the carboxylic acid is trifluoroacetic acid.
 8. A process according to claim 7 additionally comprising the step of reacting the product of claim 7 with an alkaline metal carbonate in aqueous alkanol.
 9. A process according to claim 1 which comprises sequentially reacting 21-acetoxy-3 Beta -hydroxy-5 Alpha -pregnane-11,20-dione with: a. trifluoroacetic acid in the presence of triphenyl phosphine and diethyl azodicarboxylate, and b. sodium carbonate in aqueous methanol.
 10. A process according to claim 5 which comprises the sequential steps of reacting 5 Alpha -androstane-3 Beta -ol-17-one with: a. formic acid in the presence of triphenyl phosphine and diethyl diazocarboxylate and b. potassium hydroxide in aqueous methanol. 